An automatic transmission is often provided with a hydraulic switch, which is disposed in a hydraulic circuit to each frictional engagement element for detecting a working hydraulic pressure supplied to the frictional engagement element. The widely used type of hydraulic switch is an ON-OFF switch, which is configured to output an ON signal when the operating hydraulic pressure exceeds a prescribed threshold value and output an OFF signal when the operating hydraulic pressure is less than the prescribed threshold value. In automatic transmissions, hydraulic pressure control is performed, using the signal from a hydraulic switch. Thus, a hydraulic switch malfunction assessment is executed periodically.
By the way, as is generally known, hydraulic pressure vibration occurs in the hydraulic circuit of an automatic transmission. In the event that the working hydraulic pressure, fluctuating due to such hydraulic pressure vibration (hydraulic pulsation), overlaps with the threshold value for switching between an ON state and an OFF state of the hydraulic switch, an ON state and an OFF state of the signal outputted from the hydraulic switch are undesirably repeated. That is, the “chattering” occurs. When the hydraulic-switch chattering is occurring, there is a possibility that a state of the working hydraulic pressure is erroneously detected, thus exerting a great influence on hydraulic pressure control.
To cope with this, a technology that excludes or eliminates the signal outputted from a hydraulic switch only in an unstable state where the working hydraulic pressure is vibrating or pulsating, has been disclosed. For instance, Patent document 1 teaches a technology in which it is determined that the working hydraulic pressure of a frictional engagement element has reached a target hydraulic pressure, only when the signal from a hydraulic switch has been continuously detected in a temporally continuous state. This permits hydraulic pressure control to be stably performed.
The controller, to which the signal from the hydraulic switch is inputted, is configured to signal-process the signal from the hydraulic switch every predetermined arithmetic operation cycles. Therefore, in the event that hydraulic-switch chattering has occurred due to hydraulic pressure vibration (hydraulic pulsation) occurring in the hydraulic circuit of the automatic transmission, it is difficult to accurately process the signal from the hydraulic switch undesirably chattering, depending on certain timing (at a certain time or time interval) of the hydraulic-switch chattering.
For instance, when the hydraulic switch is repeatedly outputting an On signal and an OFF signal due to chattering, but the period (the time interval) of the ON signal and the OFF signal is shorter than the arithmetic operation cycle of the controller, there is a possibility that the controller erroneously processes that the signal from the hydraulic switch is constantly kept in an OFF state depending on the timing. Therefore, even when the hydraulic switch itself is normal, there is a possibility that it is assessed or determined that a malfunction of the hydraulic switch occurs. Such an erroneous assessment may produce a great influence on hydraulic pressure control of the automatic transmission. Thus, it would be desirable to avoid the erroneous assessment.
As a countermeasure against an erroneous assessment, it is considered to specify a running condition and an operating region in which hydraulic-switch chattering may occur and then inhibit an assessment on a malfunction of the hydraulic switch from being carried out under the specified running condition and/or in the specified operating region. However, there is a possibility that hydraulic pressure vibration (hydraulic pulsation), which may cause chattering, occurs without depending on the specified running condition and/or the specified operating region. Hence, as discussed above, it is difficult to specify a running condition and/or an operating region in which hydraulic pressure vibration is predicted to occur.